Extractive desulfurization of fuel oils with low-viscosity dicyanamide-based ionic liquids

Abstract

Four low-viscosity ionic liquids (ILs) based on the dicyanamide anion ([N(CN)2]−), i.e., 1-butyl-3-methylimdazolium ([BMI][N(CN)2]), 1-ethyl-3-methylimdazolium ([EMI][N(CN)2]), ethylated tetrahydrothiophenium ([S2][N(CN)2]) and ethyldimethylsulfonium ([EtMe2S][N(CN)2]), have been investigated to determine their extraction capability for thiophene (TS) and dibenzothiophene (DBT) from model fuel oils. Aromatic imidazolium is more efficient than cyclic thiophenium and tetrahedral trialkylsulfonium; specifically, the S-extraction ability follows the order [BMI][N(CN)2] > [EMI][N(CN)2] > [S2][N(CN)2] > [EtMe2S][N(CN)2], with DBT being more efficiently extracted than TS. The S-extraction of [BMI][N(CN)2] has been investigated as a representative with respect to the influence of extraction temperature, IL:oil mass ratio, initial S-content, multiple extractions and reusability, along with its mutual solubility in oil. The percentage of S-removal from gasoline and diesel fuel were 48.5 and 68.7%, respectively, in a single extraction at 25 °C, 1:1(w/w) IL:oil, 5 min; the S-content in gasoline decreased from 599 ppm to 4 ppm after 5 extraction cycles and in diesel fuel decreased from 606 ppm to an undetectable value after 4 cycles. The mutual solubility is not pronounced and the extraction efficiency is not conspicuously changed after 6 regeneration cycles. It is worth noting that a short extraction time of < 5 min is observed for all the ILs at room temperature, which is understood by their low viscosities and effective mass transfer. This work may offer a new option for the deep desulfurization of fuel oils.